Electrified vehicles employ an electric drive system designed to provide emissions-free propulsion with reduced fuel consumption while also providing uncompromised vehicle performance. To produce a satisfying and enjoyable driving experience for an operator, the vehicle's electric drive system must reliably deliver required torque during all operating conditions. Various types of electric drive control methods can be employed to achieve this objective, including those that rely on some form of feedback. Current-feedback control systems use current sensors to provide feedback information regarding the current flow through the electric motor stator windings. Feedback can be provided to a torque control system configured to control current to the motor. When current sensors are accurate, proper drive signals can be provided and torque demand can be satisfied. However, errors at current sensors can generate current control errors that cause torque output to deviate from a command torque. While some degree of torque deviation can be tolerated without noticeable effect on performance, significant deviation can disturb electric drive operation.
Electric drive engineers are tasked to design systems having sufficient torque accuracy for satisfactory vehicle performance in all operational states. The torque accuracy required for satisfactory performance changes with torque levels and motor speed. For example, while 4% accuracy may be sufficient at torques below 50 Nm, a 7% accuracy may be required at torques above 50 Nm. While torque accuracy can depend on several factors, sensor accuracy is of particular importance in those systems using current feedback control techniques. In many cases, the need to produce vehicles that perform well at all speeds prompts an engineer to incorporate high accuracy sensors to minimize the probability that errors will occur. Current sensors can be expensive, with sensor price correlating directly with sensor accuracy. Over-specifying the sensor accuracy required for a system may unjustifiably increase system costs. Unfortunately, as of yet there is no systematic approach for determining a minimal sensor accuracy requirement for a desired torque accuracy. Sensors that fail to operate with sufficient accuracy adversely affect vehicle performance, while sensors that operate with overly high accuracy unnecessarily drive up costs.
While some prior art systems and methods have attempted to address various issues related to electric machine torque control, they have failed to isolate, determine, predict, compensate or utilize the effects of current sensing errors on estimated torque output.